As shown in FIG. 1, it is a schematic block diagram of a conventional receiver 100 with a programmable gain amplifier 105. The receiver 100 includes an antenna 101, a low noise amplifier (LNA) 102, a frequency converter 103, a channel selection filter (CSF) 104 and a programmable gain amplifier (PGA) 105.
The receiver 100 receives the ultra-high signal by the antenna 101 and the low noise amplifier (LNA) 102 amplifies the received signal. The frequency converter 103 down-converts the frequency of the received signal. The channel selection filter (CSF) 104 filters away the interference signal of the received signal. The programmable gain amplifier (PGA) 105 composed of amplifiers amplifies the received signal and outputs a signal at a predetermined value.
The receiver 100 further includes a radio frequency automatic gain control (RF AGC) 107 and an intermediate frequency automatic gain control (IF AGC) 108. The radio frequency automatic gain control (RF AGC) 107 automatically adjusts the gain of the low noise amplifier (LNA) 102 and the frequency converter 103 to maintain the ultra-high signal before being inputted to the channel selection filter (CSF) 104 at a predetermined value. The intermediate frequency automatic gain control (IF AGC) 108 automatically adjusts the gain of the channel selection filter (CSF) 104 and the programmable gain amplifier (PGA) 105 to maintain the signal after being outputted from the channel selection filter (CSF) 104 at a predetermined value.
While the signal inputted to the programmable gain amplifier (PGA) 105 is too low, the programmable gain amplifier (PGA) 105 increases the gain for the signal under the control of the intermediate frequency automatic gain control (IF AGC) 108. While the signal inputted to the programmable gain amplifier (PGA) 105 is too high, the programmable gain amplifier (PGA) 105 decreases the gain for the signal to maintain the signal at the predetermined value.
Please refer to FIG. 2. It is a schematic view of a conventional programmable gain amplifier (PGA) 105 implemented by operational amplifiers. The programmable gain amplifier (PGA) 105 includes a first operational amplifier 201, a second amplifier 202, and a plurality of resistors R201, R202, and R203.
The transfer function of the programmable gain amplifier (PGA) 105 is represented as formula 1 (E1):
                                          V            o                                V            i                          =                                            R              202                        ⁢                          R              204                                                          R              201                        ⁢                          R              203                                                          (        E1        )            
where Vo=Vop−Vom, Vi=Vip−Vim, and the programmable gain amplifier (PGA) 105 performs mapping for the gain which is inversely proportional to the input signal in the communication system. In the programmable gain amplifier (PGA) 105, the high level signal is corresponding to low gain, and low level signal is corresponding to high gain so that the outputted signal is maintained at the predetermined value. The frequency bandwidth, noise characteristic, 1 dB output gain compression point, and 3rd output intercept point (OIP3) meet the specification requirement of the communication system.
Generally, if the programmable gain amplifier (PGA) 105 has to be operated in a high gain status, the operational amplifier with high gain bandwidth product is required in order to satisfy the requirement of predetermined bandwidth. Conversely, if the programmable gain amplifier (PGA) 105 has to be operated in a low gain status, the operational amplifier with low gain bandwidth product can be used to satisfy the requirement of specific bandwidth. Please refer to FIG. 5. It is a schematic view of the relationship between the gain and frequency bandwidth of the conventional programmable gain amplifier (PGA) 105. The relationship between the gain and frequency bandwidth (BW) of the programmable gain amplifier (PGA) 105 is represented as formula 2 (E2), and the gain is inversely proportional to the frequency bandwidth (BW):
                    Gain        =                  1          BW                                    (        E2        )            
For the same operational amplifier, the frequency bandwidth (BW) is decreased when the gain of the programmable gain amplifier 105 is increased. For the design purpose of characteristic requirement, specifically bandwidth, of the programmable gain amplifier (PGA) 105 in a communication system, the programmable gain amplifier (PGA) 105 is constructed as the following design rule. In the worst case, the programmable gain amplifier (PGA) 105 is designed on the basis of the high gain rule first, and the gain bandwidth product is then determined.
For the purpose of high gain bandwidth product, the greater current is provided to the operational amplifiers thereby the programmable gain amplifier (PGA) 105 is operated in the status of high gain. Even though the programmable gain amplifier (PGA) 105 is only required to be operated in the status of low gain, or the outputted signal of the programmable gain amplifier (PGA) 105 is maintained at the predetermined value, the greater current is always provided to the operational amplifiers, thereby resulting in excessive power consumption with respect to the receiver 100.
Notwithstanding the programmable gain amplifier (PGA) 105 has to be operated in high gain in only a few cases that the inputted signal is in very low level, it is necessary to provide the greater current for the programmable gain amplifier (PGA) 105 all the time. In addition, the gain of the programmable gain amplifier (PGA) 105 is designed by a gain margin so that the greater current is provided for the programmable gain amplifier (PGA) 105 all the time. Therefore, it is required to supply more current to the programmable gain amplifier (PGA) 105, thereby resulting in a problem of unnecessary power consumption.